1. Field of the Invention
The invention pertains to the field of semiconductor devices. More particularly, the invention pertains to light-emitting diodes, wavelength-stabilized semiconductor edge-emitting and surface-emitting lasers, optical amplifiers, photodetectors, and mode-locked lasers.
2. Description of Related Art
A prior art semiconductor diode laser, or more specifically, edge-emitting laser, is shown in FIG. 1(a). The laser structure (100) is grown epitaxially on an n-doped substrate (101). The structure further includes an n-doped cladding layer (102), a waveguide (103), a p-doped cladding layer (108), and a p-contact layer (109). The waveguide (103) includes an n-doped layer (104), a confinement layer (105) with an active region (106) inside the confinement layer, and a p-doped layer (107). The n-contact (111) is contiguous with the substrate (101). A p-contact (112) is mounted on the p-contact layer (109). The active region (106) generates light when a forward bias (113) is applied. The profile of the optical mode in the vertical direction z is determined by the refractive index profile in the z-direction. The waveguide (103) is bounded in the lateral plane by a front facet (116) and a rear facet (117). If a special highly reflecting coating is put on the rear facet (117), the laser light (115) is emitted only through the front facet (116).
The substrate (101) is formed from any III-V semiconductor material or III-V semiconductor alloy. For example, GaAs, InP, GaSb. GaAs or InP are generally used depending on the desired emitted wavelength of laser radiation. Alternatively, sapphire, SiC or [111]-Si is used as a substrate for GaN-based lasers, i.e. laser structures, the layers of which are formed of GaN, AlN, InN, or alloys of these materials. The substrate (101) is doped by an n-type, or donor impurity. Possible donor impurities include, but are not limited to S, Se, Te, and amphoteric impurities like Si, Ge, Sn, where the latter are introduced under such technological conditions that they are incorporated predominantly into the cation sublattice to serve as donor impurities.
The n-doped cladding layer (102) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is doped by a donor impurity. In the case of a GaAs substrate (101), the n-doped cladding layer is preferably formed of a GaAlAs alloy.
The n-doped layer (104) of the waveguide (103) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is doped by a donor impurity. In the case of a GaAs substrate, the n-doped layer (104) of the waveguide is preferably formed of GaAs or of a GaAlAs alloy having an Al content lower than that in the n-doped cladding layer (102).
The p-doped layer (107) of the waveguide (103) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is doped by an acceptor impurity. Preferably, the p-doped layer (107) of the waveguide is formed from the same material as the n-doped layer (104) but doped by an acceptor impurity. Possible acceptor impurities include, but are not limited to, Be, Mg, Zn, Cd, Pb, Mn and amphoteric impurities like Si, Ge, Sn, where the latter are introduced under such technological conditions that they are incorporated predominantly into the anion sublattice and serve as acceptor impurities.
The p-doped cladding layer (108) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), transparent to the generated light, and doped by an acceptor impurity. Preferably, the p-doped cladding layer (108) is formed from the same material as the n-doped cladding layer (102), but is doped by an acceptor impurity.
The p-contact layer (109) is preferably formed from a material lattice-matched or nearly lattice matched to the substrate, is transparent to the generated light, and is doped by an acceptor impurity. The doping level is preferably higher than that in the p-cladding layer (108).
The metal contacts (111) and (112) are preferably formed from the multi-layered metal structures. The metal contact (111) is preferably formed from a structure including, but not limited to the structure Ni—Au—Ge. Metal contacts (112) are preferably formed from a structure including, but not limited to, the structure Ti—Pt—Au.
The confinement layer (105) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is either undoped or weakly doped. The confinement layers are preferably formed from the same material as the substrate (101).
The active region (106) placed within the confinement layer (105) is preferably formed by any insertion, the energy band gap of which is narrower than that of the substrate (101). Possible active regions (106) include, but are not limited to, a single-layer or a multi-layer system of quantum wells, quantum wires, quantum dots, or any combination thereof. In the case of a device on a GaAs-substrate, examples of the active region (106) include, but are not limited to, a system of insertions of InAs, In1-xGaxAs, InxGa1-x-yAlyAs, InxGa1-xAs1-yNy or similar materials.
One of the major shortcomings of the edge-emitting laser of the prior art is the variation of the energy band gap with temperature resulting in an undesirable temperature dependence of the wavelength of emitted light, particularly for high output power operation. Another shortcoming is a broad beam divergence.
FIG. 1(b) shows schematically a prior art surface-emitting laser, particularly, a vertical cavity surface-emitting laser (VCSEL) (120). The active region (126) is put into a cavity (123), which is sandwiched between an n-doped bottom mirror (122) and a p-doped top mirror (128). The cavity (123) includes an n-doped layer (124), a confinement layer (125), and a p-doped layer (127). Bragg reflectors each including a periodic sequence of alternating layers having low and high refractive indices are used as a bottom mirror (122) and a top mirror (128). The active region (125) generates light when a forward bias (113) is applied. Light comes out (135) through the optical aperture (132). The wavelength of the emitted laser light from the VCSEL is determined by the length of the cavity (123).
The layers forming the bottom mirror (122) are formed from materials lattice-matched or nearly lattice matched to the substrate (101), are transparent to the generated light, are doped by a donor impurity, and have alternating high and low refractive indices. For a VCSEL grown on a GaAs substrate, alternating layers of GaAs and GaAlAs or layers of GaAlAs having alternating aluminum content preferably form the mirror (122).
The n-doped layer (124) of the cavity (123) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is doped by a donor impurity.
The p-doped layer (127) of the cavity (123) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is doped by an acceptor impurity.
The layers forming the top mirror (128) are formed from materials lattice-matched or nearly lattice-matched to the substrate (101), are transparent to the generated light, are doped by an acceptor impurity, and have alternating high and low refractive indices. For a VCSEL grown on a GaAs substrate, alternating layers of GaAs and GaAlAs or layers of GaAlAs having alternating aluminum content preferably form the mirror (128).
The p-contact layer (129) is formed from a material doped by an acceptor impurity. For a VCSEL grown on a GaAs substrate, the preferred material is GaAs. The doping level is preferably higher than that in the top mirror (128). The p-contact layer (129) and the metal p-contact (112) are etched to form an optical aperture (132).
The confinement layer (125) is formed from a material lattice-matched or nearly lattice-matched to the substrate (101), is transparent to the generated light, and is either undoped or weakly doped. The confinement layers are preferably formed from the same material as the substrate (101).
The active region (126) placed within the confinement layer (125) is preferably formed by any insertion, the energy band gap of which is narrower than that of the substrate (101). Possible active regions (126) include, but are not limited to, a single-layer or a multi-layer system of quantum wells, quantum wires, quantum dots, or any combination thereof. In the case of a device on a GaAs-substrate, examples of the active region (126) include, but are not limited to, a system of insertions of InAs, In1-xGaxAs, InxGa1-x-yAlyAs, InxGa1-xAs1-yNy or similar materials.
The active region (126) generates optical gain when a forward bias (113) is applied. The active region (126) then emits light, which is bounced between the bottom mirror (122) and the top mirror (128). The mirrors have high reflectivity for light propagating in the normal direction to the p-n junction plane, and the reflectivity of the bottom mirror (122) is higher than that of the top mirror (128). Thus, the VCSEL design provides a positive feedback for light propagating in the vertical direction and finally results in lasing. The laser light (135) comes out through the optical aperture (132).
One of the major advantages of a VCSEL is the temperature stabilization of the wavelength if the device operates in a single transverse mode. Temperature variations of the wavelength follow the temperature variations of the refractive index, which are an order of magnitude smaller than the variations of the semiconductor band gap energy. A severe disadvantage of a VCSEL is that its output power is limited to a few milliwatts, because it is not possible to provide efficient heat dissipation in the VCSEL geometry keeping a single transverse mode operation, and there is a difficulty in providing a high power output density suitable for the frequency conversion. Another disadvantage of a VCSEL is that the wavelength is defined by the cavity thickness giving only a little flexibility to the device.